Chabazite-type zeolites are zeolites having a three-dimensional pore structure constructed of 8-membered oxygen rings of 3.8×3.8 Å, and are designated and classified as the structure type code CHA, as zeolites having a fully identified crystal structure, by International Zeolite Association (non-patent document 1).
Chabazite-type zeolites are known as naturally occurring zeolites and typically have the composition Ca62+[Si24Al12O72] (non-patent document 2). Examples of synthetic zeolites of the chabazite type include zeolite D disclosed in patent document 1 and zeolite R disclosed in patent document 2, each zeolite having an SiO2/Al2O3 ratio of 3.45-4.9.
In patent document 3 and patent document 4 are disclosed a zeolite of the so-called high-silica chabazite type having an SiO2/Al2O3 ratio of 5-50, which is designated as SSZ-13, and methods for synthesizing the zeolite.
In patent document 5 is disclosed a chabazite-type zeolite having an SiO2/Al2O3 ratio of 20-50 and a crystal diameter of 0.5 μm or less, which is designated as SSZ-62. Furthermore, patent document 6 and non-patent document 3 disclose that a chabazite-type zeolite having an SiO2/Al2O3 ratio of 100 or greater can be synthesized by adding fluorine.
Synthetic zeolites have regularly arranged pores having a uniform size which are due to the crystal structure thereof. Using the pore structures, synthetic zeolites are industrially used as desiccants, adsorbents for adsorbing various inorganic or organic molecules on the basis of differences in polarity and molecular diameter, solid acid catalysts, etc. For example, chabazite-type zeolites also are known to be usable as catalysts for chemically converting an oxygenic organic compound, e.g., an alcohol, into a lower olefin (patent document 6 and patent document 7). Other uses thereof which have been disclosed include an adsorptive separating agent for purifying 1,1,1,2-tetrafluoroethane (HFA134a) contained in a mixture of HFA134a and 1-chloro-2,2-difluoroethylene (HCFC 1122) (patent document 8), an adsorptive separating agent for separating the propylene from the propane by adsorbing propylene on the zeolite from a mixture of propylene and propane (patent document 9), an adsorptive separating agent for separating oxygen, carbon dioxide, or helium from a mixture thereof with nitrogen (patent document 10), a catalyst for synthesizing a methylamine compound (patent document 11), a catalyst for producing acetonitrile through ammoxidation of ethane (patent document 12), a catalyst for selective reduction of nitrogen oxides with hydrocarbon in automobile exhaust gas (patent document 13), an odor absorbent for use in fibrous members constituting sanitary articles (patent document 14), etc.
As described above, chabazite-type zeolites are expected to be utilized in a variety of application, in particular, as adsorbents and catalyst supports. However, for industrial uses, the zeolites must have durability required of adsorbents or catalyst supports. For example, for use in an adsorption/desorption process involving a thermal regeneration step, the zeolites are required not to decrease in adsorption performance even when repeatedly heated. Meanwhile, the catalysts for use in exhaust gas purification are required to have thermal durability which enables the catalysts to retain their catalytic performance at high temperatures. There has hence been a desire for a chabazite-type zeolite which has even better durability and heat resistance that have not been obtained so far.